The Trusted Firmware-A (ATF) is a transparent bare-metal application layer executed in Exception Level 3 (EL3) on the APU. The ATF includes a Secure Monitor layer for switching between the secure and the non-secure world. The Secure Monitor calls and implementation of Trusted Board Boot Requirements (TBBR) makes the ATF layer a mandatory requirement to load Linux on the APU on Versal devices. The PLM loads the ATF to be executed by the APU, which keeps running in EL3 awaiting a service request. The PLM also loads U-Boot into the DDR memory to be executed by the APU. The DDR memory loads the Linux OS in the SMP mode on the APU. The ATF (bl31.elf) is built, by default, in PetaLinux. You can find it in the PetaLinux project images directory.